A circulating fluidized bed (CFB) reactor can function as a heavy oil cracker, combustor or gasifier to process carbonaceous materials. For all of these applications, the reactor typically comprises similar components, namely a riser, a particulate collections system, a standpipe and a solids flow control valve. In addition, the reactors used for these various applications also operate in a similar manner, namely by: providing an upward flow of gas-solids mixture in the riser; collecting particles from the gas and conveying the collected particles to the standpipe; and gravitationally flowing the collected particles in the standpipe and particles flowing through a valve located in the bottom of the standpipe to the bottom of the riser.
For most CFB reactors, feed material is added to the lower portion of the riser whereby the gas and solids mixture flows upward at velocities varying from 25-70 ft/s. The gas-solids mixture then exits at the top of the riser and moves through a conventional particulate collection system. To that end, it is highly desirable to have the particulate collection system operate with the highest efficiency possible and with minimum erosion of the wall liners of the collection apparatus.
Broadly speaking, three types of gas-solid separation devices are in practice in conventional CFB systems: a gravity separator, such as disengager; a flow impactor, such as U-beams and finned tube separators; and a centrifugal separator or centrifugal cyclone. Both the gravity separator and the flow impactor are suitable for separating particles with diameters larger than 100 microns from the gas stream. The essential advantage provided by these two types of separators is reduced erosion on the walls resulting from high concentrations of large particles. In contrast, the erosion on a wall of a centrifugal separator or centrifugal cyclone often becomes a serious concern because of larger particle sizes. Hence, to achieve a balance between a minimum erosion and maximum collection efficiency, typical particulate collection systems comprise a combination of an upstream disengager or flow impactor and a downstream centrifugal cyclone. This combination allows for the removal of larger particles and a significant reduction in particle loading before entering the cyclone.
Although the gravity separator used in tandem with a centrifugal cyclone can take advantage of the characteristics of both devices, the integration faces serious challenges too. For example, the large footprint and vessel size of the disengager as well as the increased cost incurred are some of the challenges encountered when integrating these systems.
More specifically, since a gravity separator utilizes the acceleration of gravity to allow solid particles to settle away from a gas stream, the drag exerted on the particles by gas must be much less than that by the gravity to separate the particles from the gas stream. Roughly speaking, the superficial gas velocity in the gravity separator should therefore be less than 1 ft/s to separate particles with diameters larger than 100 microns. As a result, the diameter of an efficient disengager typically needs to be on the order of 3-9 times larger than that of a riser, which itself has an inner diameter of 4-6 ft for most commercial reactors. Thus, the larger the disengager must be, the more expensive it will be for both fabrication and transportation. Further, because the particle collection efficiency is generally low in the disengager, the particle concentration in the carrying gas may still be relatively high at the centrifugal cyclone inlet and may still cause significant erosion problems on the wall of the cyclone.
Typically, high particulate collection efficiency in a cyclone is intimately associated will high erosion rates on the wall because the solids and gas are spun on the same surface at a high rate of speed. Thus, it presents another challenge as to how to utilize the cyclonic effect without the resulting erosion of the cyclone wall. To date, all efforts have been directed to finding the most erosion resistance materials to prolong the refractory liner of the centrifugal cyclone.
Still another challenge for using the above noted combination of the disengager and cyclone is the method of transporting the fines collected in the cyclone to the standpipe. For relatively low temperature operations of about 1000° F. in the FCC system, the disengager also functions as the housing for cyclones and a dipleg is connected to the bottom of the cyclone and discharges the solids to a fluidized bed inside the disengager vessel. In such applications, special care is required to make certain that solids flow through the dipleg is smooth, without stoppage either from gas flowing upwards in the dipleg to the cyclone or from the solids packing in the cyclone dipleg. This has proven to be a difficult task. For example, U.S. Pat. No. 6,569,317 B1 teaches the use of a trickle valve installed in the solids exit in the dipleg to prevent gas from flowing upward in the dipleg. However, this configuration becomes impractical for gasification and combustion operations of a CFB, where the solids collected in the cyclone are at temperatures in the range of about 1600-1900° F.
Furthermore, in the latter CFB application, the circulating particles are typically ash from fuel. These particles are typically irregular in shape and vary during the operations with the fuel fed. To transport these particles under high temperatures and high pressures, the cyclone and the disengager are typically designed and operated as separate vessels with the cyclone's gas inlet connected to the disengager's gas outlet. The particles collected in the cyclone are then recycled to the standpipe through various non-mechanical valves. Since the particles collected in the cyclones generally have the top size particles of 100 μm and the mass mean particle size less than 30 μm, the bulk density of these small particles are much lower than that of the larger particles and the static head of the same height of the bed materials is also lower. Hence, to overcome the same pressure difference, the bed height or the dipleg height has to be excessively tall. Furthermore, due to the small size of particles, the particles tend to have a low setting velocity and therefore the diameter of the cyclone dipleg must be excessively large. Both these factors will increase the cost of the collections system.
Accordingly, what is needed but unavailable in the art, is an improved method and apparatus for separating particulate material from a particulate laden gas-solid process stream. In particular, there is a need for an improved apparatus and method that can enable efficient separation of relatively large particles from particulate laden gas solid stream without encountering the aforementioned disadvantages and challenges that exist in the conventional apparatuses and methods described above.